1. Field
One or more embodiments described herein relate to a display device.
2. Description of the Related Art
An organic light emitting diode display includes a plurality of pixels, each of which includes an organic emission layer between two electrodes. In operation, electrons from one of the electrodes and holes from the other electrode combine in the organic emission layer to generate excitons for emitting light.
Each pixel may also include uses a plurality of transistors and at least one capacitor to control light emission. The transistors include a driving transistor and a plurality of switching transistors. These transistors drive an organic light emitting diode, which includes the organic emission layer, based on various control signals.
In operation, the organic light emitting diode may emit light that changes from a darker level to a lighter level (e.g., from black to white) according to driving current Id received by the organic light emitting diode. The magnitude of the driving current is controlled by the driving transistor based on a gate voltage. The difference between gate voltages that correspond black and white levels of light may be referred to as the gate-voltage driving range.
Organic light emitting diode (OLED) displays of the aforementioned type have a number of drawbacks. For example, pixel size tends to decrease as screen resolution increases. A decreased pixel size may cause the amount of current flowing in each pixel to decrease, which, in turn, narrows gate-voltage driving range of the driving transistor. A narrower gate-voltage driving range may reduce the number of gray scale levels each pixel is able to emit and/or increases the difficulty of controlling the gate voltage for purposes of producing a desired gray scale level.
Further, hysteresis may occur as a result of an off-bias state corresponding to emission of a dark (or black) level of light and on-bias state corresponding to emission of a lighter (e.g., white) level of light. This hysteresis may limit the response speed of the display device in a way that adversely affects luminance, especially in the case where pixel luminance changes between darker and lighter light levels.
Further, a full HD (FHD, e.g., 1920×1080 pixels) OLED display may use one or more dual-gate transistors to reduce off current. However, in a UHD OLED display, when the size of the pixel is decreased, for example, to 800PPI (Pixel Per Inch), it may be difficult to use dual-gate transistors, because the area of the storage capacitor in each pixel is reduced and the off current may be further decreased to an insufficient level.